Groundwater Height

Although SWAT+ does not currently print groundwater height in the output files, the water table height is updated daily by the model. Groundwater height is related to groundwater flow by equation 2:4.2.6.

Qgw=8000KsatLgw2hwtbl=8000μ1010KsatμLgw2hwtbl=800μαgwhwtblQ_{gw}=\frac{8000*K_{sat}}{L^2_{gw}}*h_{wtbl}=\frac{8000*\mu}{10}*\frac{10*K_{sat}}{\mu * L^2_{gw}}*h_{wtbl}=800*\mu*\alpha_{gw}*h_{wtbl}

2:4.2.19

where QgwQ_{gw} is the groundwater flow into the main channel on day ii (mm H2_2O), KsatK_{sat} is the hydraulic conductivity of the aquifer (mm/day), LgwL_{gw} is the distance from the ridge or subbasin divide for the groundwater system to the main channel (m), hwtblh_{wtbl} is the water table height (m), μ\mu is the specific yield of the shallow aquifer (m/m), and αgw\alpha_{gw} is the baseflow recession constant. Substituting this definition for QgwQ_{gw} into equation 2:4.2.9 gives

hwtbl,i=hwtbl,i1exp[αgwΔt]+wrchrg(1expαgwΔt)800μαgwh_{wtbl,i}=h_{wtbl,i-1}*exp[-\alpha_{gw}*\Delta t]+\frac{w_{rchrg}*(1-exp\lfloor-\alpha_{gw} *\Delta t\rfloor)}{800*\mu*\alpha_{gw}} 2:4.2.20

where hwtbl,ih_{wtbl,i} is the water table height on day ii (m), hwtbl,i1h_{wtbl,i-1} is the water table height on day i1i-1 (m), αgw\alpha_{gw} is the baseflow recession constant, Δt\Delta t is the time step (1 day), wrchrgw_{rchrg} is the amount of recharge entering the aquifer on day ii (mm H2_2O), and μ\mu is the specific yield of the shallow aquifer (m/m).

Table 2:4-1: SWAT+ input variables used in shallow aquifer calculations.

Variable NameDefinitionFile Name

GW_DELAY

δgw\delta_{gw}: Delay time for aquifer recharge (days)

.gw

GWQMN

aqshthr,qaq_{shthr,q}: Threshold water level in shallow aquifer for base flow (mm H2_2O)

.gw

ALPHA_BF

αgw\alpha_{gw}: Baseflow recession constant

.gw

REVAPMN

aqshthr,rvpaq_{shthr,rvp}: Threshold water level in shallow aquifer for revap (mm H2_2O)

.gw

GW_REVAP

βrev\beta_{rev}: Revap coefficient

.gw

RCHRG_DP

βdeep\beta_{deep}: Aquifer percolation coefficient

.gw

GW_SPYLD

μ\mu: Specific yield of the shallow aquifer (m/m)

.gw

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